Don't be so sure. HTTP is stateless, so there's no concept of
logout or disconnect. Once you've delivered a page to the
client, you have no idea if that client's human is still paying
attention to the page (e.g. reading it) or if they've gone to
lunch.
Any site you see showing these kinds of figures are based on
somebody's idea of how long to wait after a page is requested
until declaring the user is no longer "on" the system; if another
request is made from the same IP before the time period expires,
the "user" is still considered "logged on"... All highly speculative,
and only you can judge whether it makes sense to do this for
your site or not.
